Method of forming interference fits by heat treatment

ABSTRACT

An interference fit can be provided between metal components where at least one of the components is formed of a precipitation-hardenable metal alloy which undergoes a substantially permanent dimensional change in addition to a reversible dimensional change upon heat treatment at elevated temperatures. The interference fit itself will be irreversible if both components are formed of the preciptitation-hardenable alloy and reversible if only one of the components is formed of the alloy.

United States Patent 13,579,805

[72] Inventor Howard Berdolt Kast 3,098,662 7/1963 lversen 285/18Fairfield, Ohio 3,316,129 4/1967 Token et al..... 148/135 [2]] Appl. No.742,882 3,408,178 10/1968 Meyers et a1. 148/142X giled d J y 3 32FOREIGN PATENTS atente ay Assignee General Electric p y 475,244 1 1/1937Great Britain ..29/(SHPT)D1g.

[54] METHOD OF FORMING INTERFERENCE FITS BY (SKFIDigest); 285/18, 381;148/4, 135, 127, 142

[56] References Cited UNITED STATES PATENTS 2,647,847 8/1953 Black etal. ..29/.(SI-1FT)Dig.UX

Primary Examiner-Charlie T. Moon AttorneysDerek P. Lawrence, E. S. Lee,111, Lee H. Sachs,

Frank L. Neuhauser, Oscar B. Waddell and Melvin M. Goldenberg ABSTRACT:An interference fit can be provided between metal components where atleast one of the components is formed of a precipitation-hardenablemetal alloy which undergoes a substantially permanent dimensional changein addition to a reversible dimensional change upon heat treatment atelevated temperatures. The interference fit itself will be irreversibleif both components are formed of the preciptitationhardenable alloy andreversible if only one of the components is formed of the alloy.

METHOD OF FORMING INTERFERENCE FITS B Y HEAT TREATMENT The inventiondescribed and claimed in the US. Pat. application herein resulted fromwork done under US. Govemment contract FASS-66-6. The US. Government hasan irrevocable, nonexclusive license under. said application to practiceand have practiced the invention claimed herein, including the unlimitedright to sublicense others to practice and have practiced theclaimed'invention for any purpose whatsoever.

FIELD OF THE INVENTION The present invention relates to a method forforming an interference fit between metal components and moreparticularly to a method wherein at least one of the components isformed of a metal alloy which undergoes dimensional change upon heattreatment thereof and wherein at least a portion of the dimensionalchange is substantially permanent.

DESCRIPTION OF THE PRIOR ART The formation of interference fits betweenmetal parts is a technique which has been practiced for many years.There are a number of widely used, relatively standard methods whichhave come into common acceptance. Primarily these methods entail themachining of the outer diameter of a male or internal member to aslightly greater dimension than the internal diameter of a female orexternal member and then either heating the external member or coolingthe internal member and interfitting the members to effect theinterference fit. More specifically in the process wherein the externalmember is heated, the member expands sufficiently to permit the positioning of the internal member therein and then cools and shrinks totightly engage the internal member. In the other technique, the internalmember is cooled sufficiently to cause it to shrink to a size which canbe easily positioned within the external member. The subsequent returnof the internal member to ambient causes it to expand and tightly engagethe external member.

In another similar well known technique the-inner member may be cooledwhile simultaneously the outer-member is heated. Thus the inner membercontracts while the outer member expands. The members are broughttogether, the inner inserted within the outer, and the assemblypermitted to equalize in temperature. Upon equalizing, the inner memberexpands, the outer member contracts and the interference fit is therebyeffected.

The foregoing well known techniques, although widely applicable, havecertain serious drawbacks. First, since the members'are interfrttedwhile at least one of them is in the expanded or contracted condition, apremium is placed upon the rapid relative positioning of the members toassure that they are properly positioned when the expected return tooriginal dimensions occurs. Thus the serious danger exists of seizingexpensive parts before they can be positioned relative to each other.Another serious disadvantage of these techniques is that the individualmembers are each subjected to differing thermal environments and thusresidual stresses remain in each member and ultimately appear in theassembly.

In response to these drawbacks, methods were developed whereby bothcomponents were simultaneously heated or cooled thus relieving thestress problem, and wherebythe positioning of the components could beeffected while each was at room temperature, thus eliminating theseizing problem. These newer techniques relied in large part upon thecharacteristic of several ferrous alloys that cooling of the alloy tosubzero temperatures, particularly in the range of 50 to 100 F, wouldcause an expansion of the alloy, and that such expansion would remainwhen the alloy returned to normal temperatures.

Specifically US. Pat. No. 2,647,847 teaches that the inter-- nal membermay be formed of this ferrous alloy while the external member is formedof a metal having more conventional expansion properties. The internaldiameter of the outer member is machined to a dimension larger than theouter diameter of the internal member and the components are positionedrelative to one another in the desired configuration. Both the internaland external members are then cooled to subzero temperatures whereuponthe internal member permanently expands while the external membertemporarily contracts. Upon returning to normal temperatures, theexternal member returns to its original dimension however the permanentexpansion of the internal member causes the interference fit.

Still another technique utilizing the freezing" principle is disclosedin'U.S.v Pat. No. 3,098,662. This technique builds upon the advantagesinherent in prior methods but additionally permitsthe use of similaralloys for both the intemaland external members. According to thismethod both'members are formed of an alloy which expands upon coolingand remains at the expanded dimensions upon return to normaltemperatures. The external member is preexpanded by exposing it tosubzero temperatures to increase the dimensions size of the bore.However, the internal member will expand at the low temperatures to formthe interference fit.

Although, apparently many of the prior art defects were overcome bythese. latter techniques, it has been found that the formation ofinterference fits at the low temperatures required by thesetechniques isundesirable. This is particularly so because treatment at lowtemperature is likely to introduce stress crackingdue to the lowductility of the components. In additionassemblies treated at these lowtemperatures generally have to be subsequently heat treated atelevatedtemperatures prior to use in order to relieve stresses and toprecipitation-harden;

SUMMARY OF THE INVENTION It is, therefore, an object of the presentinvention to provide an improved method for the formation ofinterference fits which does not require that-the components be exposedto subzero temperatures.

It is another object of this invention to provide an improved methodwhereby the drawbacks of prior 'art techniques, such as seizing prior toproper positioning and the existence of hamrful stresses in thecomponents can be eliminated.

It is still another object of this invention to provide an improvedtechnique wherein the requirement for stress relieving and precipitationhardening at elevated temperatures subsequent to the formation "of theinterference fit can be eliminated and these steps integrally includedinto the method of forming the interference fit.

Other objects and advantages will become apparent from the followingdescription and appended claims.

In accordance with the objects of this invention a method for forminginterference fits between interfitting components is provided whereindimensional changes in the components.

are brought about by heat treatment at elevated temperatures. At leastone of the components is formed of a precipitation hardenable alloywhich undergoes a substantially irreversible dimensional change as wellas a reversible dimensionalchange upon heat treatment. By substantiallyirreversible dimensionaldimensions, assembling the components and thensubjecting the assembly to a similar precipitation-hardening heattreatment to alter the dimensions of the previously untreated componentand to form the interference fit. The previously treated component willremain unchanged during the assembly heat treatment. Since thecomponents are constructed of the same alloy the interference fit isirreversible because each component will expand or contract in preciselythe same manner as its mate regardless of the thermal conditions towhich it may be exposed.

In another embodiment of this invention a reversible interference fit isprovided by forming just one of the components from theprecipitation-hardenable alloy and forming the other froma conventionalalloy which undergoes only reversible expansion or contraction upon heattreatment. The components are assembled and the assembly is subjected toa precipitationhardening heat treatment to cause a substantiallyirreversible dimensional change in the precipitation-hardenable alloyand a corresponding expansion in the conventional alloy. Upon coolingthe precipitation-hardenable alloy retains the substantiallyirreversible portion of its dimensional change and the conventionalalloy returns to its original size. The interference fit has beeneffected by the substantially irreversible dimensional change of theprecipitation-hardenable alloy. However, unlike the irreversibleinterference fit, upon heating to a temperature above the solutioningtemperature of the precipitation-hardenable alloy, the dimensionalchange therein can be reversed and the components separated.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be betterunderstood from the following description taken in conjunction with theaccompanying drawings:

FIG. 1 is a fragmentary sectional view illustrating the relativepositioning of the internal and external members prior to interfittingto form an irreversible interference fit;

FIG. 2 illustrates the members of FIG. I interfitted to form anassembly;

FIG. 3 illustrates a portion of FIG. 2 greatly exaggerated to show theclearance between members;

FIG. 4 is a fragmentary sectional view illustrating the relativepositioning of the internal and external members prior to interfittingto form a reversible interference fit utilizing aprecipitation-hardenable alloy which contracts upon heat treatment; and

FIG. 5 is a fragmentary sectional view illustrating the relativepositioning of the internal and external members prior to interfittingto form a reversible interference fit utilizing aprecipitation-hardenable alloy which expands upon heat treatment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In providing the interferencefits of the present invention at least one of the components, and insome instances both, are formed of a precipitation-hardenable alloywhich retains a dimensional change when subjected to heat treatment atelevated temperatures. Such dimensional change is uniform along all axesof the alloy and is unrelated to and independent of the thermalcoefiicient of expansion. It is a known property of mostprecipitation-hardenable alloys that they will undergo such adimensional change, either expansion or contraction in going from thesolution-annealed and cooled condition to the precipitation-hardenedcondition. The extent and nature of that dimensional change varies fromalloy to alloy and is de pendent upon the conditions of the heattreatment. The most important feature in connection with these types ofdimensional changes is that the changes are substantially irreversibleor substantially permanent. Because the alloy is dimensionally stableafter heat treatment an article made from the alloy will not return toits original dimensions upon cooling or other heat treatment below thesolutioning temperature of the alloy. This behavior is cruciallydifferent from that of conventional alloys which expand upon heating orcontract upon cooling but which will return to their original dimensionswhen returned to normal temperatures.

The term precipitation-hardening heat treatment, as used herein, means aconventional heat treatment by well known techniques at temperatures andfor times which will bring about the desired precipitation of hardeningconstituents but below the solutioning temperature. This temperaturewill vary from alloy to alloy but in all cases is a value which iseasily ascertainable by one skilled in the art. Heat treatment in therange above the minimum temperature necessary to initiate suchprecipitation will generally increase the extent of the dimensionalchange. However, in many cases, with such as a stainless steel sometimesidentified as I7-4PH, beyond a particular temperature where austeniteforms, the increased dimensional change will be achieved at the expenseof hardness and strength. Thus the choice of a precipitation-hardeningheat treatment temperature will require consideration of desiredultimate properties as well as the desired dimensional change.

Examples of alloys well known in the art and which are known to undergosubstantially pennanent dimensional change upon heat treatment includel7-7 PH, l7-4PH, PH 15-7 M, l55 PH and the nickel base super alloyslnconel 718, lnconel X and lnconel W.

In order to utilize the advantageous characteristic of these alloys thatthey experience a substantially permanent dimensional change, it ispreferable first to solution anneal components constructed of thesealloys. An example of a typical alloy is 17-4 PH stainless steelconsisting nominally by weight of 0.8 Si, 16 Cr, 4 Ni, 0.3 Cb Ta, 3 Cuwith the balance Fe and impurities. The solution anneal for such alloyshould be accomplished at temperature of from about l,875 -1,925 F. forabout 1 hour. The structure of the alloy of this temperature ispredominantly austenite with any hardening elements, e.g., copper,dissolved therein. Generally close control of the solution anneal isdesirable because temperatures which are too low result in reducedtensile and yield strengths and incomplete solution of the hardeningelements. Excessively high temperatures cause excessive grain growth andresult in lower tensile ductility and reduced impact strength. Thesolutioned alloy is then air cooled or oil quenched, in accordance withknown techniques, to to transform the austenite to martensite. Themartensite may then be aged or precipitation-hardened at temperatures inthe range of from about 900 to l,150 F. for at least 1 hour andpreferably from l,0501,l00 F. l7-4 PH steel contracts markedly in thesetemperature ranges, the substantially irreversible contractions rangingfrom about 0.0004 to 0.0006 inches per inch depending upon thetemperature. The higher the temperature the greater such contraction.Also longer exposures at hardening temperatures will increase suchcontraction. I

The method of forming the interference fit can be better understood byreference to FIG. 1 wherein a body member 10 is shown having a bore 13therein said bore having an inner diameter A. The body member mayalternatively be referred to as an outer member, female member, orsleeve member. An inner member 14, also referred to as a male member orspool member, is shown having an outer diameter B. The members areadapted to form an assembly 15 by interfitting inner member 14 withinthe bore 13 of outer member 10, as can be seen more clearly in FIG. 2.

In one embodiment of the present invention, an irreversible interferencefit is provided. Both members are formed of an alloy which isprecipitation-hardenable and which undergoes substantially irreversibledimensional change. These members are solution annealed at a temperatureabove the solutioning temperature of the alloy and cooled to normalambient temperatures. Both members are, at this point, prepared toundergo a precipitation-hardening heat treatment in order to takeadvantage of the accompanying dimensional change.

The method of forming the interference fit will depend upon whether thealloy being employed contracts or expands upon heat treatment. If it isa contracting alloy, for example 17-4h steel, the inner or male membercan be precipitationharde'ned in accordance with well known techniquesto alter its dimensions. It is particularly desirable that all heattreating be accomplished in a vacuum or inert atmosphere to preventoxidation or corrosion. The internal diameter A of the outer member isadjusted as necessary to be just slightly larger than outer diameter Bof the already dimensionally altered inner member. When components and14 are interfitted to form an assembly 15, as shown in FIG. 3, theclearance a between the members should be smaller than the expectedsubstantially irreversible contraction of the outer member.

The precise clearance is a matter of choice. It is clear that the largerthe clearance the easier it will be to interfit the members. On theother hand the smaller the clearance the tighter the resultinginterference fit will'be. The assembly 15, consisting of contractedinner member 14 housed within the bore 13 of outer member I0 is thensubjected to a precipitation-hardening heat treatment in a mannersimilar to the heat treatment employed for the inner member alone. Sincethe inner member is precontracted, its dimensions will not changesubstantially irreversibly during this second hardening treatment.However, outer member 10 will contract substantially irreversibly toform the interference tit and a tightly interfitting assembly.

In a similar manner an irreversible fit can be formed if the alloychosen is one which expands substantially irreversibly when subjected toa precipitation-hardening heat treatment, e.g., 17-7 PH steel. In thiscase, after solution annealing, the outer member isprecipitation-hardened to expand and permanently alter its dimensions.The diameters A and B are adjusted to permit the members to beinterfitted with a small clearance, as hereinbefore discussed. Theinterfitted assembly 15 consisting of expanded outer member 14containing inner member 10 within bore 13 thereof is then subjected to aprecipitation-hardening heat treatment. The preexpanded outer memberwill not change irreversibly its dimensions during this treatment butthe inner member will undergo a substantially irreversible expansion toform the interference fit and a tightly interfitting assembly.

The tightly interfitting assemblies thus far described, wherein thecomponents thereof are each fonned of the precipitation-hardenablealloy, are irreversible: the members cannot be separated because boththe inner and outer members will undergo precisely the same reversibleexpansion or contraction in response to any thermal conditions. Evenshould the assembly be heated above the solutioning temperature of thealloys to retrieve the substantially permanent dimensional change, bothalloys would tend to return to their original dimensions in preciselythe same manner.

In another embodiment of the present invention, as can be seen in FIGS.4 and 5, a reversible interference fit can be provided by forming onlyone of the members of the precipitation-hardenable alloy which undergoessubstantially permanent dimensional change. The other component, in thisembodiment, is formed of a conventional alloy which expands on heatingbut returns to its original dimension on return to normal ambienttemperatures. FIG. 4 shows the members forming a reversible interferencefit with a contractingalloy. The outer member 17 is formed of theprecipitation-hardenable alloy and inner member 19 is formed of aconventional alloy. To form the interference fit, the outer member isfirst solution-annealed and cooled, the members are interfitted withinner member 19 housed within the bore 18 of outer member 17. A slightclearance is provided between inner diameter C of outer member 17 andouter diameter D of inner member 19, and the assembly is subjected to aprecipitationhardening heat treatment, preferably in an inertatmosphere. During the high-temperature treatment outermember 17substantially permanently contracts due to metallurgical changes andexpands due to thermal expansion and inner member 19 made of theconventional alloy only expands. Upon cooling,

outer member I7 retains a portion of its dimensional change but innermember 19 returns to its pre-heat-treatment dimension. If the clearanceswere originally chosen correctly, the substantially permanentcontraction will be sufficient to effect the interference fit.

This interference fit, in contrast to those previously discussed isreversible. This is true because the members are formed of dissimilaralloys and each will respond differently to heat treatment. For exampleif the assembly is annealed above the solutioning temperature thesubstantially permanent dimensional change in the outer member will berelieved, the contraction will be eliminated and the members will hebecome separable. Since separation requires such extreme temperatures,the assembly is useful in applications over a very wide range oftemperatures without any real danger of the members accidentally orinadvertently separating.

Still another technique for forming a reversible interference fit can beexplained with reference to FIG. 5 wherein the inner member 22 is formedof a precipitation-hardenable alloy' which expands upon heat treatmentand the outer member 20 is made of a conventional alloy. Theinterference fit is formed by solution-annealing and cooling innermember 22, interfitting that inner member within the bore 21 of outermember 20 with a slight clearance provided between inner diameter E andouter Diameter F. Upon subjecting the interfit assembly to aprecipitation-hardening heat treatment, inner member 22 willsubstantially permanently expand due to thermal expansion plusmetallurgical changeswhile outer member 20 will also expand due tothermal expansion. Upon cooling outer member 20 will return to itspre-heat-treatment dimensions and the interference fit will have beeneffected in a manner similar to that described in connection with FIG.4.

In all of the techniques described for forming the interference fits ofthis invention, a primary advantage is that the members were interfittedto form the assembly while they were at normal ambient temperatures.This eliminates the problems associated with seizing and the pressuresof rapid positioning, In each case, however, the step of interfittingthe components carries the limitation that the outer diameter of theinner member and the inner diameter of the outer member must have aslight clearance there between. Under no circumstances should theclearance exceed the expected net dimensional change since it should beclear that in such a case no intel-ference fit will be formed.

The following examples are illustrative of the present invention.

Example I An irreversible interference fit was formed by providing innerand outer members of 17 -4PI-I stainless steel and subjecting them to asolution-anneal for about 1 hour at about l,900 F. and then air coolingthe members to room temperature. The inner member was subjected to aprecipitationhardening heat treatment at about 1,075" F. for about 1hour to cause the member to contract. The inner member was theninterfitted within the bore of the outer member and a small clearance ofabout 0.0003 inches was provided. The entire assembly was subjected to aprecipitation-hardening heat treatment at about l,075 F. for about 1hour. All heat treatments were accomplished in an argon atmosphere.

The resulting tightly interfitted assembly was tested by applying 4,400lbs. of force in an attempt to separate the members. The interferencefit withstood the applied force with no resulting motion.

Example 2 A reversible interference fit was formed by providing an outermember of l7-4PH stainless steel and an inner member of M2 tool steel.The outer member was solution-annealed at 1,900 F. for 1 hour and aircooled to room temperature. The inner member was interfitted within thebore of the annealed outer member and the interfitted assembly was heattreated at about l,075 F. for about 1 hour. All heat treatments wereaccomplished in an argon atmosphere.

The tightly interfitted assembly was tested at 4,400 lbs. force and nomovement was noted indicating that the interference fit had been formed.

To demonstrate the reversible nature of the fit, the assembly wassolution-annealed at about l,900 F. for about 1 hour. A force of only695 lbs. was needed to start separation and, thereafter, the partsseparated easily indicating that the anneal had relieved the dimensionalcontraction of the outer member.

While the present invention has been described with reference toparticular embodiments thereof, it will be understood that numerousmodifications can be made by those skilled in the art without actuallydeparting from the scope of the invention.

lclaim:

l. A method for forming an interference fit between inner and outermembers adapted to be interfitted, comprising the steps of:

a. providing at least one of said members of a precipitationhardenablealloy which undergoes substantially irreversible dimensional change uponheat treatment thereof,

b. solution annealing said precipitation-hardenable alloy at atemperature in excess of the solutioning temperature of said alloy,

0. interfitting said members to form an assembly with a clearancebetween said members smaller than the substantially irreversibledimensional change, and

d. subjecting said assembly to a precipitation-hardening heat treatmentwhich causes said substantially irreversible dimensional change.

2. A method, as claimed in claim 1, wherein said precipitation-hardeningheat treatment is accomplished at a temperature of from about 900? to1,150 F. I

3. A method, as claimed in claim 1, wherein said members are both formedof said precipitation-hardenable alloy and step of subjecting said innermember to a precipitationhardening heat treatment aftersolution-annealing thereof.

6. A method, as claimed in claim 4, wherein said dimensional change isan expansion and including the additional step of subjecting said outermember to a precipitation-hardening heat treatment after solutionannealing thereof.

7. A method, as claimed in claim 1 wherein one of said members is formedof said precipitation-hardenable alloy, the other of said members isformed of a conventional alloy, and said resulting interference fit isreversible.

8. A method, as claimed in claim7, including the additional step aftersolution-annealing of said precipitation-hardenable alloy of coolingsaid alloy prior to interfitting of said members.

9. A method, as claimed in claim 8, wherein said dimensional change is acontraction and said outer member is formed of said alloy.

10. A method, as claimed in claim 8, wherein said dimensional change isan expansion and said inner member is formed of said alloy.

1. A method for forming an interference fit between inner and outermembers adapted to be interfitted, comprising the steps of: a. providingat least one of said members of a precipitationhardenable alloy whichundergoes substantially irreversible dimensional change upon heattreatment thereof, b. solution annealing said precipitation-hardenablealloy at a temperature in excess of the solutioning temperature of saidalloy, c. interfitting said members to form an assembly with a clearancebetween said members smaller than the substantially irreversibledimensional change, and d. subjecting said assembly to aprecipitation-hardening heat treatment which causes said substantiallyirreversible dimensional change.
 2. A method, as claimed in claim 1,wherein said precipitation-hardening heat treatment is accomplished at atemperature of from about 900* to 1,150* F.
 3. A method, as claimed inclaim 1, wherein said members are both formed of saidprecipitation-hardenable alloy and said interference fit isirreversible.
 4. A method, as claimed in claim 3, including theadditional step after solution annealing of saidprecipitatioN-hardenable alloy of cooling said alloy prior tointerfitting of said members.
 5. A method, as claimed in claim 4,wherein said dimensional change is a contraction and including theadditional step of subjecting said inner member to aprecipitation-hardening heat treatment after solution-annealing thereof.6. A method, as claimed in claim 4, wherein said dimensional change isan expansion and including the additional step of subjecting said outermember to a precipitation-hardening heat treatment after solutionannealing thereof.
 7. A method, as claimed in claim 1 wherein one ofsaid members is formed of said precipitation-hardenable alloy, the otherof said members is formed of a conventional alloy, and said resultinginterference fit is reversible.
 8. A method, as claimed in claim 7,including the additional step after solution-annealing of saidprecipitation-hardenable alloy of cooling said alloy prior tointerfitting of said members.
 9. A method, as claimed in claim 8,wherein said dimensional change is a contraction and said outer memberis formed of said alloy.
 10. A method, as claimed in claim 8, whereinsaid dimensional change is an expansion and said inner member is formedof said alloy.